This invention relates to an assembly fixture and method used for the fabrication of metal grids. It relates particularly to apparatus and a method used for the fabrication of metal grids used to support and position the fissionable fuel rods used in a nuclear reactor core.
Most nuclear reactors use as fuel, elongated rods of fissionable material arranged and supported in a spaced parallel array between upper and lower core support plates. To provide integrity within the supports, the fuel rods are divided into groups and the fuel rods in each group are formed as an integral fuel rod assembly prior to placement between the reactor core support plates.
More specifically, the fuel rods in each group have been typically in the past arranged in spaced, parallel arrangements with each other in supporting and spacing frames or grids. The grids are formed of a plurality of interconnected metal grid straps to provide a square or rectangular structural network of interconnected open grid cells, similar to an "egg crate divider".
U.S. Pat. Nos. 3,379,617 and 3,379,619 issued Apr. 23, 1968 to Andrew et al. and assigned to the assignee of the present invention, disclose typical supporting grid assemblies for fuel rods in which the fuel rods are held laterally and longitudinally in the grid cells by resilient, spring-like retainers in selected grid cells of the grid structure.
In the past the assembly of the grids has been difficult and time consuming due to the large number of grid strap components, their small size, the flexibility of the individual grid straps and the large number of welds required to connect all the grid straps and components together into an integral structural grid network.
In addition, the nuclear fuel assembly grid specifications require that the that the grid straps and other grid components be very accurately aligned, both during and after assembly and welding, in order to hold the group of fuel rods making up a fuel rod assembly in a precise, parallel alignment. Such accurate alignment was often difficult and time consuming to achieve with the previously known multi-step grid assembly and welding fixtures and practices used prior to this invention.